Final answer:
An intrusion detection system monitors network activities for suspicious incidents and alerts network security personnel of possible intrusions or misuse.
Step-by-step explanation:
A software and/or hardware device that monitors system and network resources and activities, and notifies network security personnel when it identifies possible intrusions from outside the organization or misuse from within the organization is a(n) intrusion detection system (IDS). Unlike an intrusion prevention system (IPS), which not only detects but also prevents intrusions, an IDS does not actively block the intrusion but rather alerts the appropriate parties. An anti-virus device is specialized in detecting and removing malicious software, and a network monitor primarily observes and manages network traffic without necessarily providing intrusion detection functionalities.